linux-kernel - Re: [RFC PATCH 1/3] Unified trace buffer

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Date:	Thu, 25 Sep 2008 10:07:17 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Ingo Molnar <mingo@...e.hu>, Martin Bligh <mbligh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Martin Bligh <mbligh@...igh.org>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Tom Zanussi <zanussi@...cast.net>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer

On Thu, 25 Sep 2008, Steven Rostedt wrote:
> 
> Slight correction. You can annotate the function with "notrace" and 
> that function will not be traced. So the "only be disabled on a per-file
> basis" statement is false.

Ok. It's still true that we absolutely don't want to add random notrace 
markers to code just because it's shared with the scheduler. And 
"sched_clock()" is not a single function with just a few well-defined 
places, nor are all versions of it at all appropriate for tracing (the 
non-TSC ones are a total joke - it works for scheduling, but not tracing. 
Same goes for the virtualized versions).

> Currently my code calls "ring_buffer_time_stamp" to get the time stamp, 
> whatever it will be.  Currently it is using sched_clock, but since I have 
> it as a wrapper, it shouldn't be too hard to modify later.

Yes. The code looked fine, and had a FIXME. I have no objection to using 
it as a known buggy approximation for TSC in order to not force every 
architecture to immediately write one when the patch is discussed. But I 
literally would expect that on x86, we'd basically just have a function 
that does "rdtsc" for the common case, along with possibly a generic 
fallback that does "xadd" in the absense of any other reasonable 
alternative.

				Linus
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